It is known that various blood parameters may be calculated by measuring the transmittance of light at different wavelengths through tissue having blood flowing therein. Examples of such blood parameters include carbon monoxide, carbon dioxide, glucose, and oxygen concentrations. Accurate information on these blood parameters may be important for a variety of reasons. For example, in the operating room, up-to-date information regarding oxygen saturation can be used to signal changing physiological factors, the malfunction of anaesthesia equipment, or physician error. Similarly, in the intensive care unit, oxygen saturation information can be used to confirm the provision of proper patient ventilation and allow the patient to be withdrawn from a ventilator at an optimal rate.
The non-invasive technique of measuring the transmittance of light in order to formulate blood parameter information is desirable in many applications for reasons of operator convenience and patient comfort. One well-known technique is pulse transmittance oximetry. This technique generally involves measuring the transmittance of light through body tissue at two different wavelengths. Typically, the two wavelengths are in the red and infrared regions. The measurements are made at both systolic pressure and diastolic pressure. In one known formulation, the arterial oxygen saturation ratio in given by the formula: ##EQU1## where R.sub.OS is the oxygen saturation ratio, R.sub.L is the transmittance of light at the red wavelength through the body tissue at systolic pressure, R.sub.H is the transmittance of light at the red wavelength through the body tissue at diastolic pressure, IR.sub.L is the transmittance of light at the infrared wavelength through the body tissue at systolic pressure, and IR.sub.H is the transmittance of light at the infrared wavelength through the body tissue at diastolic pressure. The actual value of oxygen saturation may be ascertained from the R.sub.OS value using empirically derived calibration curves. The precise description of the method and apparatus for measuring the transmittance of light is not part of the present invention and so is described only generally. Reference to U.S. Pat. No. 4,819,646 entitled "Feedback-Controlled Method and Apparatus for Processing Signals Used in Oximetry" to Cheung et al. is recommended for a detailed description of pulse transmittance oximetry.
Moreover, although only a formulation for the oxygen saturation ratio is given above, formulations for carbon dioxide, carbon monoxide, and other blood parameters based upon measurements of the transmittance of light are known in the art. The present invention is equally applicable to those formulations and although the following description of the preferred embodiment relates to pulse transmittance oximetry, it should not be construed to be limiting the scope of the present invention.
As seen in the equation for R.sub.OS, four parameters must be measured: R.sub.L, R.sub.H, IR.sub.L, and IR.sub.H. In any measurement technique where a physical parameter must be measured, random noise errors may arise. Noise errors in measured parameters contribute to inaccuracies in the calculated values based upon the measured parameters; in this case, error in the oxygen saturation ratio R.sub.OS. In order to more accurately calculate oxygen saturation ratios, various methods have been utilized in the prior art.
One well known and easy to implement method is averaging. In this method, multiple calculated values of R.sub.OS are averaged to provide a "smoothed" final value. Because the value of R.sub.OS is obtained as a quotient of measured parameters, it has been found that post calculation averaging does not provide optimum noise elimination. An improvement is to average only certain values of R.sub.OS. In particular, "outliers" are eliminated from the values of R.sub.OS before the average is taken. Outliers are values for R.sub.OS that are outside a predetermined range from the past history of R.sub.OS values. Again, it has been found that even with this improvement, optimum noise elimination is not obtained.
The present invention is directed towards providing an improved method of eliminating random noise error.